1. Field of the Invention
The present invention relates to an oxide ion conductor and a method of producing the same. In particular, the present invention relates to an oxide ion conductor which is preferably usable as a solid electrolyte of a fuel cell and which is composed of a composite oxide of lanthanum, a tetravalent element, and an element with which at least any one of them is substituted. The present invention also relates to a method of producing the oxide ion conductor.
2. Description of the Related Art
The solid, in which ions are movable, is widely known as an ion conductor. Also, as well known, the ion has the positive or negative electric charge. Therefore, a current flows through the ion conductor when the ions move.
The mobile ion differs depending on the type of the ion conductor. For example, β-Al2O3, which has a composition of Na2O.11Al2O3, is a good ion conductor for Na+, i.e., a sodium ion conductor, which is adopted as a solid electrolyte of the sodium/sulfur cell. On the other hand, AgI has long been known as a good silver ion conductor from old times.
Recently, because of the growing concern over environmental protection, a fuel cell is used as a low pollution electric power source. Attempts are being made to adopt an oxide ion (O2−) conductor as an electrolyte of the fuel cell. In this case, the entire fuel cell can be made of solid materials, for the oxide ion conductor itself is a solid, making the structure simple. Further, the number of times of maintenance can be reduced because no liquid leakage occurs.
A typical crystalline structure of the oxide ion conductor is the fluorite (CaF2) type structure. Examples of such structure include stabilized ZrO2 doped with about 8 mole % of Y2O3, stabilized ZrO2 doped with about 15 mole % of MgO, Bi2O3 doped with about 25 mole % of Y2O3, and CeO2 doped with about 25 mole % of Gd2O3. In particular, the two types of stabilized ZrO2 described above are practically used as a solid electrolyte of a fuel cell and a partition wall of an oxygen sensor for measuring oxygen concentration in gases or molten metals.
Examples of other oxide ion conductors are those which have the perovskite (CaTiO3) type structure; for example, La0.9Sr0.1Ga0.8Mg0.2O3 and BaTh0.9Gd0.1O3. Usage of these compounds as a partition wall, an oxygen sensor and also a thermistor, is being considered.
The oxide ion conductivity of the oxide ion conductor having the fluorite or the perovskite type structure described above is satisfactory at a high temperature range of about 800 to 1000° C. Therefore, to operate a fuel cell using the oxide ion conductor as a solid electrolyte, it is necessary to raise the temperature of a laminated stack to about 800 to 1000° C. The laminated stack comprises a power generation cell or a plurality of power generation cells electrically connected to one another to constitute the fuel cell. At a temperature lower than the foregoing temperature range, the conductivity of the sold electrolyte (oxide ion conductor) decreases, thus, markedly lowering the power generation efficiency.
When, however, the fuel cell is operated at a high temperature range as described above, a large amount of energy (electric power or the like) is required to heat the power generation cell or the laminated stack. Additionally, inexpensive metal materials, such as stainless steel cannot be used as a member for constituting the fuel cell. The mechanical strength and the corrosion resistance of such a metal material decreases at a high temperature. Therefore, operating the fuel cell at a high temperature significantly increases the running cost.
Both of Japanese Laid-Open Patent Publication Nos. 8-208333 and 11-71169 suggest an oxygen ion conductor composed of a composite oxide of rare earth element(s) and Si, and a method of producing the same. In both of the patent documents, it is described that the composite oxide exhibits excellent oxide ion conductivities at low and middle temperature range of 200 to 600° C., as compared with the two types of the oxide ion conductors described above.
Further, in Solid State Ionics 136–137 (2000 edition), pp. 31–37, studies are made on La10Si6O27, La10Ge6O27 and composite oxides obtained by substituting a part of La with Sr of the two. It is reported that these composite oxides also exhibit excellent oxide ion conductivities in the temperature range of 200 to 600° C. as compared with the two types of the oxide ion conductors described above.
Generally, to produce an oxide ion conductor composed of a composite oxide of a rare earth element and Si, the composite oxide of rare earth elements and Si are sintered at a temperature exceeding 1700° C. This is because the melting point of the foregoing composite oxide is high, and the sintering process is insufficient at a temperature lower than 1700° C. In other words, it is difficult to obtain a sintered product (oxide ion conductor) at a temperature lower than 1700° C., which is strong enough for practical use.
When, however, parts of the reactor, which are used for sintering, such as the heating element, heat insulating material and reaction tube, are heated up to a temperature exceeding 1700° C., the durability rapidly falls. That is, the life of the reactor is drastically shortened and the equipment cost is extremely expensive for producing the oxide ion conductor composed of the composite oxide of rare earth elements and Si. Consequently, the expensive production cost of the oxide ion conductor is a great drawback.
In the scientific paper described above, La10Ge6O27 and La10-XSrXGe6O27, in which a part of La is substituted with Sr, are obtained by isostatically pressing a mixed powder of GeO2, SiO2, and SrCO3 at 275 Mpa, and then sintering in a temperature range of 1600 to 1650° C.
However, large amounts of impurities such as La2GeO5 and La2Ge2O7 are contained in the products of La10Ge6O27 and La10-XSrXGe6O27 obtained from above described procedure. The oxide ion conductivities of the oxide ion conductors containing such impurities are extremely low at low through middle range temperatures as compared with the oxide ion conductivity of the pure oxide ion conductor. The conductivity of the oxide ion cannot be improved, because the impurities in the composite oxides of lanthanum and germanium having the same compositions as described above.